I. Field of the
The present invention relates to amplifiers. More particularly, the present invention relates to a novel and improved radio frequency amplifier circuit which provides linear control of gain, when such gain is measured in decibels, over a large gain range.
II. Description of the Related Art
The use of automatic gain control (AGC) circuits to control amplifier gain in communication terminals has been accomplished by many various designs. Typically the range in gain for such amplifiers is somewhat limited, and therefore are not directly useful in applications where a very large dynamic range of signals are to be accommodated.
In applications where a surface acoustic wave (SAW) filter is utilized in the receiver for band filtering, special care must be taken in choosing the impedance of the circuit seen by the SAW filter. Traditionally the SAW filter is matched in impedance with the external circuit by using simple series coupled inductors at the input and output of the SAW filter. Such simple impedance matching provides for a reduction of the well known phenomena of triple transit interference (reflections of the signal of interest within the SAW filter). However, such a reduction in triple transit is at the expense of increased insertion losses.
A typical insertion loss using a conventionally configured SAW filter is approximately 20 dB. The minimum insertion loss that a simple SAW filter is theoretically capable of producing is 6 dB. However, at the minimum insertion loss the triple transit signal is at a level that can substantially interfere with a signal of interest.
In applications requiring a large gain control, and which also utilizes SAW filters for band filtering, the loss of the SAW filter should be minimized if one is to maximize the effective range of controllable gain.
In certain applications, such as in CDMA cellular telephone transceivers where transmitter power control and receiver AGC are essential to system operation, the transmitter and receiver must both operate over a relatively high dynamic range and track each others gain change over the full range of control. In such an environment, the receiver can be required to operate over a gain range that can easily reach as high as 80 dB.
In many systems, again such as in CDMA cellular telephone transceivers, it is essential that in response to a received signal, the AGC loop should provide a signal indicative of measured signal strength which is linear, in terms of output indication as a function of dB of input. Conventional AGC circuits normally exhibit such a characteristic over only a portion of the full control range, due to nonlinearities in the gain control function of the amplifier.
It is therefore an object of the present invention to provide a novel and improved amplification circuit which, in response to a gain control signal, is capable of substantially linear gain control operation over a relatively large range.
It is another object of the present invention to provide an amplifier circuit in which gain in dB is controlled as a linear function of an input control signal.
It is yet another object of the present invention to provide active impedance matching of a SAW filter to minimize insertion loss at specified levels of triple transit signals.
It is still a further object of the present invention to provide a temperature compensated amplification circuit capable of substantially linear gain control over a large dynamic range over a wide temperature range.